1. Field of the Invention
The present invention relates generally to an apparatus and method for allocating resources in a mobile communication system, and in particular, to a resource allocation apparatus and method for efficiently transmitting Voice over Internet Protocol (VoIP) packets in a mobile communication system.
2. Description of the Related Art
Wireless communication systems have been developed to cope with the situation where it is not possible to connect a fixed wire network up to terminals. The typical wireless communication systems include not only the normal mobile communication system for providing voice and data services, but also a Wireless Local Area Network (WLAN), a Wireless Broadband Internet (Wibro), a Mobile Ad-Hoc network, etc.
Mobile communication, unlike the normal wireless communication, is premised on mobility of users. The ultimate aim of mobile communication is to enable users to exchange information and media at anytime and anyplace using Mobile Stations (MSs) such as a portable phone, a Personal Data Assistant (PDA), and a radio pager. With the rapid development of communication technology, the mobile communication system has reached a phase of providing not only the normal voice call services but also high-speed data services in which the transmission of high-volume digital data such as moving images as well as e-mail and still images is possible using the mobile terminals.
In addition, due to various user demands for high-quality services, there is an increasing need for a communication system capable of efficiently providing high-speed packet data services. In order to meet the need, many companies are looking for a new method capable of reducing the cost related to providing voice services without depriving the convenience of and familiarity with the existing services from the users. The cost reduction accelerates integration of the data network and the voice network, and a careful system design and planning is needed to prevent the integration of the data network and the voice network from affecting the quality and reliability of the voice network.
In this context, Voice over IP (VoIP) is now under discussion, which transmits voice packets over a packet network.
Traffic characteristics of the VoIP service will now be described with reference to FIG. 1.
FIG. 1 is a diagram illustrating traffic characteristics of a VoIP service in a normal mobile communication system, in which an Adaptive Multirate Codec (AMR) vocoder generates traffic. The generation period and traffic size of voice data, though they are subject to change according to the vocoder type, have the following characteristics.
A period where traffic exists is called a talkspurt period 101, while the other period is called a silent period 103. The vocoder generates packet data with a predetermined size every 20 ms 102 in the talkspurt period 101, and generates a Silent Indicator (SID) having a fixed size every 160 ms 104 in the silent period 103. While a size of the voice data is about 40 bytes even at the full rate, a size of the normal Internet data is several hundreds to several thousands of bytes. Therefore, it can be considered that the size of the voice data is significantly greater than the size of the normal Internet data.
Transmitting a Physical Downlink Control Channel (PDCCH) for transmitting scheduling information or resource allocation information at every transmission time in order to transmit small-sized voice data generated every 20 ms 102 can be significant signaling overhead. Therefore, a persistent resource allocation method or a persistent scheduling method has been proposed to efficiently support the services having the foregoing characteristics. With reference to FIG. 2, a description will now be made of the persistent resource allocation method or the persistent scheduling method.
FIG. 2 is a diagram illustrating a persistent scheduling method in a normal mobile communication system.
A terminal is allocated particular time and particular resource in the entire 20-ms period through upper layer signaling or PDCCH. In the case of FIG. 2, the terminal is allocated persistent resources in three Transmission Time Intervals (TTIs). The persistent resources can be identified through upper layer signaling or PDCCH during call setup. In the latter case, the PDCCH needs to have bit information indicating whether the corresponding resource allocation is the dynamic resource allocation or the persistent resource allocation. It can be noted in FIG. 2 that 3 TTIs 201, 202 and 203 are allocated at intervals of 5 ms for the 20-ms period using the persistent resource allocation method, and it can be appreciated that a total of 4 transmission opportunities can be given considering a Hybrid Automatic Repeat reQuest (HARQ) Round Trip Time (RTT). However, in the case of FIG. 2, the third one of the transmission opportunities is persistently allocated. Then the terminal attempts demodulation of a Physical Downlink Shared Channel (PDSCH), over which data is transmitted, at particular times 201, 202 and 203 where the resources are allocated, using information on the previously persistently allocated resources even when there is no PDCCH information transmitted to the terminal.
However, the foregoing normal persistent resource allocation method or persistent scheduling method should always persistently allocate resources as many times as the required number of transmissions, causing a reduction in the scheduling flexibility and the total resource efficiency. In order to solve this problem, a new method is now under discussion, in which a transmission side allocates resources using the persistent resource allocation method only for initial transmission, and when retransmission occurs, as it has failed to receive a Not-Acknowledge (NACK) message indicating a failure to receive data from a reception side, the transmission side performs dynamic resource allocation using PDCCH.
With reference to FIG. 3, a description will now be made of the initial transmission-limited persistent resource allocation method.
FIG. 3 is a diagram illustrating a method of using persistent scheduling only for initial transmission in a normal mobile communication system. In FIG. 3, a base station allocates resources with the persistent resource allocation method only in one TTI in the 20-ms period as shown by reference numerals 303 and 306.
A terminal can perceive that the base station allocates resources with the persistent resource allocation method always at the initial time 303 in the 20-ms period, and when the base station fails in its transmission of resource allocation information caused by the persistent resource allocation method at the initial time, the base station transmits resource allocation information or scheduling information using PDCCH 301 as shown by reference numerals 309, 310, 311 and 312. When the terminal perceives that there is resource allocation information transmitted thereto through PDCCH 301 at the transmission times 309, 310, 311 and 312, the terminal receives data 304, 305, 307 and 308 through resources indicated by the resource allocation information transmitted at the transmission times 309, 310, 311 and 312.
In FIG. 3, the base station allocates resources with the persistent scheduling method at the predetermined transmission times 303 and 306, and performs resource allocation dynamically at the transmission times 309, 310, 311 and 312, and the terminal receives data through the resources 304, 305, 307 and 308 indicated by resource allocation information for the dynamically allocated resources. However, since the method, shown in FIG. 3, which allocates resources with the persistent resource allocation method only for the initial transmission and allocates resources dynamically for the next transmissions, increases the required number of PDCCHs as the number of retransmissions at the transmission side increases, the method may also increase the signaling overhead as in the dynamic resource allocation scheme. At a low initial transmission Block Error Rate (BLER), the number of terminals performing retransmission is small. However, the normal system has difficulty in maintaining the low initial transmission BLER for the following four reasons.
1. Inaccuracy of Selecting Modulation and Coding Scheme (MCS) Level
When performing persistent resource allocation for initial transmission, the base station fixes not only the position and amount of wireless resources but also the MCS level. In the downlink, the terminal makes a decision (selection) based on received Channel Quality Indicator (CQI) information, and in the uplink, the base station makes a decision based on the pilot transmitted by the terminal. In this case, a measurement error on CQI and pilot strength may occur. When a moving velocity of the terminal is high, the measurement error will increase considerably. Since this value is not correct, it is difficult to select an MCS level so that the base station can maintain a low BLER.
2. Instability of Power Control
There is a need to maintain a constant reception power level through power control in order to maintain the scheduled MCS. To this end, it is necessary to measure CQI or pilot. However, a measurement error on the CQI or pilot occurs due to the measurement error or the velocity of the terminal, making it difficult to perform perfect power control.
3. Inter-Cell Interference
Even though an MCS level was correctly selected, if inter-cell interference measured during the decision is different from inter-cell interference during actual transmission, it is not possible to satisfy the desired BLER. Since an Orthogonal Frequency Division Multiple Access (OFDMA) system is significantly susceptible to interference compared to the Code Division Multiple Access (CDMA) system, the OFDMA system has more difficulty in maintaining a low initial BLER.
4. Power Shortage of Terminals Located in Cell Boundary
In the uplink, a terminal located in the cell boundary may not support a high data rate or a high initial transmission BLER due to a power shortage. In this case, the terminal can transmit data with low transmission power if the terminal increases an initial data rate and transmits the data after dividing it into several segments.